This invention relates generally to optical viewing devices. More particularly, this invention relates to an optical viewing device for viewing remote locations which are difficult or impossible to directly access.
Optical viewing devices of the type described herein are well known and used in a variety of applications. For example, such devices are configured as borescopes and used to view or inspect the interior of remote cavities such as engine cylinders and the like. Borescopes thus find extensive utility in the automotive and aerospace industries. In the medical field, such optical viewing devices are configured as endoscopes or laparoscopes. Endoscopes (or laparoscopes) are well known medical instruments which are used to visualize the interior of a body cavity or hollow organ. Endoscopes are used in a variety of operative procedures including laparoscopic surgery where endoscopes are used to visually examine the peritoneal cavity.
Typical prior art optical viewing devices (e.g., borescopes and endoscopes) comprise three main sections including an objective lens followed by a relay lens followed by an eye piece. The relay lens section functions to transmit the image rays obtained by the objective lens along a preselected distance to the eye piece. The relay lens may consist of either a coherent fiber optical bundle or a series of lenses. When the relay lens section is comprised of a series of lenses, the lens series traditionally includes repeated units (e.g., three) with each unit comprising a symmetric arrangement of glass rods and lens groupings. Thus, in a prior art device having three relay sections, a total of six glass rods and associated lens groupings were required. These glass rods are used to preclude or lessen divergence of the image rays as they travel through the optical viewing device. Such glass rods must be of a high optical quality and therefore the glass rods lead to a relatively high manufacturing cost for each optical viewing device. Moreoever, in certain prior art viewing devices, the objective lens and eye piece sections also contain glass rods leading to even higher manufacturing costs.
Illumination in the remote location to be viewed is required. A preferred method of transmitting illumination light is to use a bundle of optical fibers surrounding the outer circumference of the optical viewing device. These fibers are generally glass. The field of illumination should be of a size no less than the field of view (FOV) of the optical viewing device. Typically, the FOV may range between 30 to 75 degrees.
Presently, there is a need for low cost and/or disposable optical viewing devices such as borescopes and endoscopes. One method of reducing costs would be to remove the relatively expensive glass rods. However, it has been believed by those skilled in the art that removal of the multiple glass rods sacrifices optical performance in several ways. For example, under certain optical conditions, removal of the glass rods reduces the light gathering ability of the optical viewing device which leads to an overall dimmer edge to the image obtained by the optical viewing device. Also, the length of the optical viewing device must be reduced which in some procedures (e.g., surgical), may be highly undesirable. Thus, it has heretofore been believed that removal of the glass rods from the relay lens section was not a practical or desirable method of producing a lower cost optical viewing device.
The prior art has addressed this problem in U.S. patent application Ser. No. 838,602 filed Feb. 19, 1992 assigned to the assignee hereof and incorporated herein by reference. In U.S. Ser. No. 838,602 a low cost optical viewing device is provided which comprises the traditional three sections of an objective lens, relay lens and eye piece. However, in contrast to the above prior art optical viewing devices, all three sections of the present invention are free of the relatively expensive glass rods. The inventor herein has discovered that deletion of the intermediate glass rods will not sacrifice optical performance. The preclusion of the glass rods in the relay lens section leads to an overall lower cost optical viewing device (e.g., borescope and endoscope).
In addition, the optical viewing device of U.S. Ser. No. 838,602 achieves lower cost by the use of molded plastic lenses rather than the more expensive glass lenses. The method of illuminating the remote location to be viewed in U.S. Ser. No. 838,602 is accomplished by using the low cost plastic optical fibers. However, because plastic optical fibers have a relatively small field of illumination, it has been discovered that by twisting the fibers (e.g., 15 degrees), the field of illumination is increased and may be tailored to match (or be no less than) the size of the field of view of the optical viewing device.
However, the use of molded plastic lenses results in birefringence being introduced into the light. The molded plastic lenses do not have a homogeneous refractive index, this problem is inherent with injection molded plastic lenses. The nonhomogeneous refractive index of the plastic lenses is the cause of the birefringence. This problem results in blurred and/or otherwise distorted images at the eyepiece.